I just thought I'd share a recent discovery / head smacking moment.
I recently took a commercial hot tub circulation pump (Pentair Whisperflo 3/4hp) in for service. It had been very noisy since start-up, and was not putting out the same flow - last year was 47 GPM (the design rate), this year was 30 GPM. The shop found the impeller had been modified. Someone plugged two of the six ports in the impeller. Not even in balance - they plugged ports at the #1 and #3 positions, leaving #2, #4-6 open. The shop replaced the impeller with a new one and I reinstalled the pump.
Well wouldn't you know it? The pump was STILL noisy and STILL only putting out 30 GPM. ... turns out when I had an electrician install a new hand/auto switch for the pump to replace a bad on/off switch (which didn't auto-restart after an outage), he wired the 3-phase connection backwards. Corrected that and it ran quiet and fast.
Too fast. After restoring the impeller, the pump runs at 64 GPM. Way above our design rate. Seems the engineer spec'ed the pump wrong, and to get the proper flow rate somebody decided to jam up the impeller. Brilliant...